Begin by boiling a large pot of salted water. Add whole wheat pasta and cook until it's almost al dente, about 6-7 minutes. Just one minute before it’s done, add the fresh broccoli florets. Drain the pasta and broccoli together, then set them aside while you prepare the sauce.

In a large sauté pan, heat 2 tablespoons of butter or olive oil over medium-high heat. Once hot, add chopped onions and cook for about 3-4 minutes until they become softened and translucent. Next, stir in sliced mushrooms and minced garlic, sautéing until the mixture is golden and fragrant, approximately 2-3 minutes more.

Sprinkle 2 tablespoons of flour over the sautéed vegetables, stirring to create a roux. Gradually whisk in 2 cups of vegetable stock and 1 cup of milk, ensuring there are no lumps. Bring the mixture to a simmer and cook until it thickens, about 4-5 minutes, stirring frequently until the sauce coats the back of a spoon.

Once the sauce has thickened, stir in half of the sharp cheddar cheese, using a spatula to melt it smoothly into the mixture. Season with salt and pepper to taste.

In a large baking dish, combine the drained pasta, broccoli, and cooked chicken. Pour the creamy sauce evenly over the mixture, stirring gently to combine all ingredients.

Preheat your oven to 400°F (200°C) and place the assembled casserole in the middle rack. Bake for about 15 minutes, until it's bubbly and hot throughout. Remove it from the oven, sprinkle the remaining cheddar cheese on top, and return to the oven for an extra 10 minutes.

After baking, look for a beautifully browned top and bubbly edges. Let the casserole rest for a few minutes before serving.